Does this need subjunctive?

I was joking with a French friend. I wrote:

S'ils m’éclateront, je t’éclaterai.

I thought putting both in future tense was correct.

She wrote back (after a laugh) that I should use subjunctive…

S'ils m'éclatent, je t'éclaterai. 

I searched my grammar book and online some resources but I couldn't find a rule about using the subjunctive with “si”. Can you point me in the right direction?

Bonjour Todd,

This is not subjunctive. Here are a few reasons why it is not subjunctive:

1. The subjunctive is used primarily in the subordinate clause and although éclatent could be present tense indicative or subjunctive, it is not in the subordinate clause here, but rather the primary clause. 
2. Si is not used with the subjunctive, but most commonly with conditional patterns (those can use both indicative or conditional tenses)
3. Si is actually commonly used to avoid the subjunctive when expressing doubt. We can replace que with si which switches the conjugation from subjunctive to indicative.
4. The subjunctive can, on rare occassions appear as a single clause or in the primary clause, however these are restricted to third-person, literary and formal imperatives, or fixed expressions. The above example is neither of these.

The example sentence you provided should follow the pattern: present indicative + future indicative. If condition #1 is met (present), then something will occur (future). This works exactly the same as in English:

• If you drink too much, I will leave.
• If we save enough money, we will buy a car.
• If we don't leave now, we will be late.

I hope this helps,

   -   Mitchell